The present invention relates to a sheet feed device having suction cups for feeding a sheet in an image forming apparatus such as a copying machine, and more particularly to a lock mechanism for use with a sheet cassette in the image forming apparatus.
Sheet feed devices for separating and feeding sheets, one at a time, from a sheet stack generally include sheet feed rollers for supplying sheets through frictional engagement between the rollers and the uppermost sheet and also between stacked sheets. However, the sheet feed rollers may damage the sheets because of the friction, depending on the type of sheets used. Another sheet feed device includes suction cups for attracting sheets under vacuum, as disclosed in Japanese Laid-Open Patent Publication No. 55(1980)-93744, for example.
There is known a sheet feed device having a compact and simple mechanism for lifting and lowering suction cups to separate and feed sheets from a sheet cassette. The known mechanism includes an elevator arm supporting suction cups and a sheet sensor. When a sheet feeding cycle starts, the elevator arm is lowered until the sheet sensor detects the upper surface of the uppermost sheet of the sheet stack in the sheet cassette. When the sheet sensor detects the uppermost sheet, the elevator arm is stopped, and the suction cups are actuated to attract the sheet under vacuum.
As the number of sheets remaining in the sheet cassette is reduced, the elevator arm has to be lowered a longer distance. If the sheet cassette is removed while the elevator arm is in the lowered position, the sheet cassette may hit elements of the sheet feed mechanism such as the suction cups and damage them due to under forces applied to these sheet feed mechanisms.